The long-range goal of this proposal is to provide new and rational approaches aimed at facilitation of the host's immune defense against tumors. The short-range goal of the proposal is to establish variant lines of the DBA/2 mouse L1210 which express differences in the amounts or the kinds of tumor-associated antigens (TAA) and/or normal cell surface antigens and to study their immunogenicity and immunoregulatory functions. The existing drug-resistant L1210 sublines which express high amounts of the TAA will also be used. We will study (1) whether variant cells with low TAA density elicit stronger supressor cell activity than those with high TAA density, (2) whether variant cells differing in TAA elicit different types of host responses or (3) whether differences in density of normal surface antigens affect the ability of variant cells to elicit host responses. Antigenic properties of variant lines will be thoroughly analyzed with monospecific and monoclonal antibodies directed to the TAA. Such antibodies will be produced by developing hybridomas through fusion of DBA/2 spleen cells immunized against L1210 with cells of a Balb/c myeloma subline. The variant lines with differences in TAA will be established by selection with monospecific antibodies and the fluorescence activated cell sorter (FACS) or by in vitro cloning. The immunogenicity of the variant cells will be tested in vitro culture systems by stimulating syngeneic mouse lymphoid cells against the irradiated tumor cells. The antibody-forming cell (AFC) responses and cell-mediated cytotoxic (CMC) responses will be measured by the plaque assay and the 51 Cr-release cytotoxicity test, respectively, using the tumor cells as target. To assess the ability of tumor cells to elicit suppressor activity, lymphoid cells of donors properly primed by the tumor cells for suppressor cell induction will be tested for their ability to suppress the induction of the anti-tumor AFC responses or of the CMC responses in lymphoid cell cultures. The ability to elicit transplantation immunity in syngeneic mice and the susceptibility to natural resistance in certain histocompatible F1 hyprids will be tested in vivo.